1. Field of the Invention
This invention relates to chemical processes for transforming halogenated hydrocarbon gases, particularly hydrofluorocarbons manufactured for use as refrigerants, into useful salts.
2. Description of the Prior Art
Due to their damaging affect on the stratospheric ozone layer, regulation of halogenated refrigerant gases (HRGs) containing chlorine have increased. Stratospheric ozone is depleted as a consequence of ultra-violet solar radiation, which frees chlorine from HRGs to produce chlorine monoxide. This in turn reduces ozone to its precursor, oxygen. The rate of ozone replenishment, i.e. conversion of oxygen to ozone by UV radiation, is much slower than that of the ozone depletion which is currently being experienced. Depletion of the ozone layer results in increased UV exposure through the earth's surface which can have serious consequences, such as increased skin cancer in humans, reduction of immune protectiveness and increased plant and animal mutations.
Because of their environmental damage potential, disposal and reclamation of these HRGs has become increasingly important. Examples of HRGs are hydrofluorochlorocarbons, like HCFC-22, or chlorofluorocarbons, like CFC-11 and CFC-12. Halons, used as streaming agents for firefighting, are similar to HRGs, but contain bromine in addition to chlorine and fluorine.
Since the implementation of laws prohibiting the venting of HRGs into the atmosphere, HRGs like HCFC-22 are oftentimes recovered from used equipment, especially air-conditioning and heat pump systems. One of the problems associated with reclamation of HRGs is the increase of cross-contamination of HRGs which can easily occur by the accidental action of those handling these substances. Failure to identify cross-contamination can result in loss of service performance or equipment failure. Identification and appropriate separation techniques for purifying or recovering cross-contaminating refrigerant gases are complex and expensive. In some cases, cross-contaminated HRGs are sometimes diluted with uncontaminated HRGs as a disposal remedy. However, due to the small amount of cross-contamination which is acceptable, this does not provide a reasonable alternative to disposing and reclamation of cross-contaminated refrigerant gases.
The disposal of mixed HRGs, that cannot be reclaimed are typically disposed of by incineration. Typically HRGs are incinerated in cement lined kilns. It is necessary to inject a low percentage of HRGs compared to other non-HRG gases undergoing destruction to prevent destruction of the cement lining of the incineration kiln. Thermal oxidation of HRGs typically produces hydrochloric and hydrofluoric acids. These very corrosive acids attack and damage the exposed cement lining and metal fixtures of the kiln.
When HRG and halon gas is destroyed by thermal processes, it has been a standard practice for products of the thermal destruction process to be scrubbed with sodium hydroxide and the sodium salts produced are then disposed in landfills after stabilization. The water solubility of sodium fluoride is 4% by weight, compared to the water solubility of sodium chloride, which is 25% by weight. There is not sufficient economic value to afford chemical separation of sodium halide salts of this type, such as by solvent evaporation or fractional precipitation. Disposal of these mixed halide salts results in both the waste of economic resources and posses a potential environmental hazard to ground water when disposed as a sodium salt in high concentrations. To reduce the cost of destruction for HRGs, an improved process is required for destruction which does not damage the reactor and can recover economic products for commercial use. The current high cost for thermal destruction of HRGs increases the potential for owners of mixed HRGs to illegally vent these gases into the atmosphere.
At the present time, there are only two commercial processes that recover useful halogens from the destruction of HRGs and can offset the cost of destruction by the reuse or sale of economic products. One process was first developed by Hoechst AG, Frankfurt, Germany. A description of the process is found in European Patent No. 0212410. The Hoechst process uses a hydrogen and oxygen combustion reaction to first heat disposed HRGs to oxidize carbon and release chlorine and fluorine gases. The gases are hydrated and fractionated to produce hydrochloric and hydrofluoric acids and chlorine gas. Carbon dioxide is vented to the atmosphere by the Hoechst process and is not recovered. Hoechst refers to their process as "reactor cracking."
DuPont Chemicals, Wilmington, Del., has also developed a process similar to Hoechst that produces useful hydrofluoric acid that can be used by DuPont for the production of commercial chemicals including HRGs that do not deplete the ozone layer. The DuPont process converts hydrochloric acid to calcium chloride that can be sold. While it is practical for users of hydrofluoric and hydrochloric acid to reclaim these toxic and very corrosive acids for their own use, the technology employed is not applicable to wide deployment in light industries associated with the reclamation of HRGs and halons.
The US-EPA has promulgated rules for both destruction and transformation of controlled substances like HRGs under the Clean Air Act Amendments of 1990, Title 6, Subpart 82, "Protection of Stratospheric Ozone," (amendments published in 40 CFR Chapter 1, page 340, Jul. 1, 199.) The US-EPA defines transformation as a process that uses and entirely consumes (except for trace quantities) a controlled substance in the manufacture of other chemicals for commercial purposes.
The percentage of carbon contained in HRGs and halons is about 10% by weight. During thermal oxidation of HRGs and halons, carbon is converted to carbon dioxide, which is not a trace product of the reaction. As a result of this definition, neither the Hoechst nor the DuPont process is a transformation process because carbon dioxide, contained in HRGs and halons, is manufactured and vented to the atmosphere.